1. Field of the Invention The invention relates to electronic speed regulators for internal combustion engines and in particular to throttle actuators for such regulators.
2. Background of the Art
The precise speed control of internal combustion engines is desired for many applications but is particularly important when such engines are used to drive AC generators. The speed of the engine determines the frequency of the generated power and many AC powered electrical devices require accurately regulated AC frequency. In addition, this accurate speed control must be maintained under rapid load variations which may result from nearly instantaneous changes in the consumption of electrical power from the generator. Variation in engine speed with change in engine load is termed "droop".
Engine speed control may be performed by a number of methods. A mechanical governor may sense the speed of rotation of the engine and open or close the throttle to regulate the engine speed in response to imputed load changes. Such mechanical control has the advantage of being relatively inexpensive, but may allow substantial droop during normal load variations.
More sophisticated engine speed control may be realized by sensing engine speed electronically and using an electro-mechanical actuator connected to the throttle to change the throttle position.
Typically, the electro-mechanical actuator is a linear or rotary actuator. As the names imply, a linear actuator has a control shaft which extends from the body of the actuator and moves linearly by a distance proportional to the magnitude of a current or voltage applied to the actuator. A rotary actuator has a shaft which rotates by an angle proportional to the magnitude of the applied current or voltage. In both actuators, a spring returns the shaft to a zero or "home" position when no voltage or current is applied to the actuator. The power consumed by these actuators is increased by the return spring whose force must be overcome.
Neither the linear nor rotary actuators may be connected directly to the rotating throttle. In the case of a linear actuator, a pitman arm must be used to convert the linear motion of the actuator to the rotary motion necessary to rotate the throttle valve through approximately 90.degree.. For a rotary actuator which rotates approximately 15.degree.-20.degree. a "four-bar" linkage is required to increase the angular motion of its shaft. The power of the actuators must be sufficient to overcome the friction associated with these required mechanical linkages.
The power required by the use of a return spring and by the friction of the mechanical linkages increases the cost and weight of a throttle control using linear or rotary actuators. For these reasons, it is known to use a bidirectional stepper motor in place of a linear or rotary actuator for the purpose of electronic engine control.
A bidirectional stepper motor is an electro-mechanical device that moves a predetermined angular amount and direction in response to the sequential energization of its windings. When a bidirectional stepper motor is used to control the throttle, the return spring may be omitted or made weaker allowing the use of a smaller motor with equivalent or better dynamic properties than the linear or rotary actuators. Also, the digital nature of the stepper motor's input signal is well adapted for use with certain microprocessor based engine controls.
The use a lower powered bidirectional stepper motor requires that the connection between the stepper motor and the throttle valve is free of binding and unnecessary friction. The throttle shaft normally fits closely within the throttle body and as a result of the fuel saturated environment, operates without lubrication. The design of the stepper motor also requires that the motor shaft have little play to preserve the close tolerances of the internal magnetic gaps for maximum power. Accordingly, in order to prevent the binding of these shafts without the introduction of excessive rotational play, the stepper motor shaft and throttle shaft are typically joined by means of the four bar linkage used with a rotary actuator. A four bar linkage comprises a connecting rod attached by pivoting joints to two cranks, one crank attached to the throttle shaft and one to the stepper motor shaft. The fourth bar is implicit in the common mounting of the motor and throttle. This linkage provides an inexpensive and easily manufactured connection between the stepper motor shaft and the throttle shaft but one that accepts some misalignment.
The connecting rod of the four-bar linkage also permits the displacement of the stepper motor away from the throttle shaft to permit the attachment of a position feedback device to the throttle shaft. A position feedback device permits the measurement of absolute throttle position which is not determinable from the control inputs to the stepper motor, because the stepper motor may start in any position.
There are two disadvantages to the use of a four bar linkage to connect the stepper motor to the throttle shaft. First, the rotational range of the stepper motor is unnecessarily limited as the four bar linkage has a limited rotational range. Second, a feature of such a linkage is that the torque transmitted by the connecting rod varies markedly depending on the relative angles of the cranks to the connecting rod. Typically at the extremes of travel there is a "dead center" position where the linkage is ineffective. However, the transmission of torque is not constant at any angle. This problem is usually addressed by making the linkage adjustable so that the crank and connecting rod angles are centered to provide peak torque transmission at the angles appropriate for a particular throttle. This solution, however, requires that the linkage be adjustable or redesigned for different throttle and engine types.